1. Field
The present invention is in the field of glass manufacturing and is principally concerned with materials and methods for the finishing of glass sheets for electronic information displays.
2. Technical Background
There is presently a very high demand for glass sheet exhibiting improved properties for use in advanced information displays for televisions, computer display screens, mobile phones, laptop and tablet computers, media players, and other electronic devices. Among the glass properties required by manufacturers and users of such devices are light weight, high resistance to damage from impacts and flexural stresses, good resistance to surface damage from scratching and other abrasion, and excellent optical quality. Desirable optical properties include freedom from light-scattering surface or internal flaws and, in many cases, a surface finish that can reduce glare and otherwise improve the optical quality of displayed images.
The need for light weight dictates that glass sheet of slight thickness be used, e.g., glass below 2 mm in thickness and more commonly below 1 mm or even 0.5 mm in thickness. Meeting the high physical durability requirements in glass sheet in these thickness ranges requires that some form of glass strengthening be used. Thus tempering of the sheets to develop surface compression layers that increase the resistance of the glass sheets to stress breakage is commonly employed, with the presenting preferred tempering method involving chemical tempering, including so-called ion-exchange strengthening.
While ion-exchange strengthening is capable of developing very high surface stress levels, and thus very high flexural strengths, in thin sheet glass sheets of appropriate composition, further improvements in the resistance of the sheets to impact and abrasion damage have become necessary as the thicknesses of the sheets continue to be reduced. Statistical studies of impact breakage variability in thin strengthened glass sheets have determined that surface quality variations, more particularly variable surface flawing resulting, for example, from the handling of the sheets during manufacture, play a role.
A recently discovered solution to the problem of impact strength variability is the use of a light surface etching treatment to moderate surface flaw behavior. Etching can also be useful to impart anti-glare characteristics to the surfaces of the sheets and/or to imprint a logo or other marking on sheet surfaces. However, such etching treatments must be carefully controlled in order to avoid undesirable surface damage or degradation of surface optical properties.
Among the etching media found useful for improving the properties of display glass sheet are aqueous fluoride solutions optionally comprising additional mineral acids. These are typically applied to the surfaces of the sheets by dipping or spraying. However, several problems relating to the use of these media and methods remain to be solved.
One problem relates to the volatility of the acid components of these etching media, e.g., HF and HCl volatility, the vaporization of these constituents acting to reduce media acid concentrations over time. In addition, where a bath of etching medium is used for dipping, the rising concentrations of dissolved glass constituents and sediments in the bath, as well as the consumption of the active fluoride etchants over time, make it difficult to maintain etching efficiency over reasonable bath service intervals.
Securing the result of uniform etching over large sheet surface areas is also impeded by the mobility of presently available etching media and methods. The low viscosities of conventional acid mixtures can result in uneven flow patterns that can cause uneven surface removal and visibly non-uniform surface optical properties. In the case of anti-glare surface finishes involving the use of patterned masks to achieve controlled differential etching rates, uncontrolled media flow can disturb weakly adhering mask materials and/or produce non-uniform anti-glare properties, including visible flow lines, light and dark spots, locally excessive haze or sparkle (as measured by pixel power standard deviation, or PPD), and/or a higher than desired DOI (distinctness of image) value. These defects affect the suitability of the etched anti-glare surface finishes for enhancing the image quality of pixelated information displays. Slow sheet immersion and withdrawal rates can minimize mask damage and improve etch patterning, but result in larger etching time differences as between the leading and trailing edges of the sheets, again affecting optical uniformity across the breadth of sheet surfaces.
The problem of unwanted etching medium flow has been addressed in the past through the use of additives to thicken and partially immobilize the medium. However, previously known formulations comprising thickeners introduce new problems that make them unsuitable for the strengthening and/or re-surfacing of optical display glasses. One problem with conventional thickening approaches is that of securing pH levels sufficiently acidic to achieve the required strengthening and other surface-enhancing effects within times sufficiently short to prevent optical damage (e.g., visible hazing or frosting) to sheet surfaces. The use of high concentrations of thickeners can dilute the effectiveness of the media and require unacceptably long etching times. On the other hand, excessively thickened media make it difficult to ensure uniform distribution and thus uniform etching of sheet glass surfaces.
Another problem relates to the instability of conventional thickeners (e.g., xanthan gum, sugars, cellulose derivatives, etc.) in highly acidic media. In particular, the oxidation of such thickeners by pH-controlling media constituents such as sulfuric acid can cause media discoloration and/or loss of thickener effectiveness.